Integrated circuits, the key components in thousands of electronic and computer products, are interconnected networks of electrical components fabricated on a common foundation, or substrate. Fabricators typically use various techniques, such as layering, doping, masking, and etching, to build thousands and even millions of microscopic resistors, transistors, and other electrical components on a silicon substrate, known as a wafer. The components are then wired, or interconnected, together to define a specific electric circuit, such as a computer memory.
Interconnecting millions of microscopic components typically follows one of two different methods, both of which initially entail covering the components with an insulative layer. In the first method, fabricators dig small holes in the insulative layer to expose portions of the components underneath and then, through metallization, the process of depositing a metal, they cover the entire insulative layer with a thin layer, or sheet, of aluminum, filling the holes with aluminum. Fabricators then apply an etch-resistant mask, which defines a wiring pattern, to the aluminum layer and subsequently etch, or dissolve, away unwanted aluminum, leaving behind an aluminum wiring pattern. The second method, known as a damascene process, entails digging trenches between the small holes in the insulative layer and then covering the entire insulative layer with aluminum, filling the holes and trenches with aluminum. Fabricators then polish off the metal outside the holes and trenches, leaving aluminum in the holes and trenches to define the wiring pattern. Both methods typically yield aluminum wires that are about one micron thick, or about 100 times thinner than a human hair.
Silicon dioxide and aluminum are the most common insulative and conductive materials used to form interconnections today. However, at submicron dimensions, that is, dimensions appreciably less than one micron, aluminum and silicon-dioxide interconnection systems present higher electrical resistances and capacitances which waste power and slow down integrated circuits. Moreover, at these smaller dimensions, aluminum exhibits insufficient electromigration resistance, a characteristic which promotes disintegration of the aluminum wires at certain current levels. This ultimately undermines reliability, not only because disintegrating wires eventually break electrical connections but also because aluminum diffuses through surrounding silicon-dioxide insulation, forming short circuits with neighboring wires. Thus, aluminum and silicon-dioxide interconnection systems waste power, slow down integrated circuits, and compromise reliability.
Several metals, such as gold, silver, and copper, appear, because of their lower electrical resistances and higher electromigration resistances, to be promising substitutes for aluminum. And, many polymeric insulators, for example, fluorinated polyimides, because of their lower dielectric constants—an indicator of how much capacitance they will introduce—appear to be promising substitutes for silicon dioxide. Lower capacitance translates into faster, more efficient integrated circuits. Thus, a marriage of these metals with polymers promises to yield low-resistance, low-capacitance interconnective structures that will improve the speed, efficiency, and reliability of integrated circuits.
Unfortunately, conventional etch-based interconnection techniques are impractical for making gold, silver, and copper interconnects. Specifically, silver, gold, and copper, are very difficult to etch. In fact, conventional attempts to etch a layer of silver, gold, or copper covered with an etch-resistant mask usually dissolve the mask faster than the gold, silver, or copper. Additionally, conventional techniques of working with polymers promote chemical reactions between the polymers and metals, such as copper, which undermine the insulative and capacitance-reducing properties of the polymers.
Accordingly, to build smaller, faster, more-efficient, and more-reliable integrated circuits, there is not only a need for new fabrication methods that work with gold, silver, and copper but also a need for methods that effectively combine these metals with the advantages of polymeric insulators.